The curing of bright-leaf (or flue-cured) tobacco is particularly energy intensive, due to the large amount of moisture which must be removed from the freshly harvested leaf. Conventional tobacco curing methods are relatively inefficient, requiring an input of heat energy approximately three times that theoretically needed for vaporizing the moisture contained in the harvested leaf. Because of the high energy requirements in the curing process and the soaring fuel costs and frequent fuel shortages, there is considerable need for a more efficient method and means for curing bright-leaf tobacco which provides for a reduction in the energy required in the curing process.
The energy crisis has created considerable interest in using solar energy for on-farm drying of grain, tobacco and other crops, and various systems using solar energy for drying grain, peanuts, and tobacco have been proposed. Previous research on solar curing of flue-cured tobacco has involved curing the tobacco in a single chamber in a conventional manner, with solar heating being utilized to assist the furnace and thereby reduce fuel requirements. However, due to uneven energy demands during the curing cycle, such systems have been unable to effectively utilize the solar energy throughout the curing cycle.
In this regard, energy demand in the tobacco curing process differs at various phases during the curing cycle. During the first phase, which typically ranges from about 36 to 72 hours, the tobacco leaves undergo yellowing, and energy demand is relatively low, with the air temperature around 95.degree.-105.degree. F. and with high air recirculation and high relative humidity. After yellowing, the drying phase begins, and energy demand increases significantly as the curing air temperature is gradually increased up to about 165.degree. to 170.degree. F. with increased fresh air intake during most of the drying phase and with reduced air recirculation. The total drying phase, including leaf and stem drying, requires about 3 to 4 days with maximum fresh air intake at chamber temperatures between about 110.degree. and 130.degree. F. Considerable sensible heat is discharged from the exhaust vent during this phase.
A solar tobacco curing system disclosed in Huang U.S. Pat. No. 4,069,593 provides for storage of excess solar energy during the yellowing phase and utilization of the collected plus the stored energy during the subsequent drying phase when energy demands are greater. While energy requirements are reduced by this system in comparison with conventional bulk curing systems, curing equipment costs are higher due to the added energy storage system. Additionally, it is recognized that the stored energy cannot be fully utilized due to various losses.
Another recent development in tobacco curing systems is disclosed in Fowler U.S. Pat. No. 4,114,288. This system is designed to increase the efficiency of a bulk curing barn by using a solar collector for preheating incoming fresh air during daytime. Additionally, under certain conditions, heat is also recovered from the heated exhaust air discharged from the barn. However, even with this system, the operation of the bulk curing barn is still relatively inefficient. The heat recovery system and solar collector are useful primarily during the drying phase of curing when substantial amounts of heated air are exhausted from the curing chamber. At other times, e.g. during yellowing or during cool down and conditioning, these systems are essentially unused and provide no energy savings. Also with this system, a considerable quantity of heat energy is lost to the atmosphere at the end of the curing cycle when the cured and dried tobacco, at an elevated temperature of 165.degree. F. or higher, is cooled to ambient temperature for removal from the curing barn.
It is an object of the present invention to provide a method and apparatus which is applicable to the curing of tobacco, and which is considerably more energy efficient than the systems heretofore proposed or commercially available.
A further object of this invention is to provide a more energy efficient curing and drying system which may also be utilized in the drying of other crops, such as corn, small grain and peanuts.